Switching devices for load branches are used, for example, in switching stations. These switching stations are usually protected from short circuits at several levels (supply, distribution, load). The current magnitude of the maximum possible short circuit at each level depends on the components of the station. From the customer's point of view, it is worthwhile to use switching devices of which the maximum permitted short circuit current is as high as possible. This simplifies planning, and avoids additional steps, which affect cost, to limit the short circuit height.
In the IEC, the behavior of the switching device is separated into two assignment types in the case of a maximum short circuit current in association with a specified short circuit protective device (circuit breaker or fuse). In the case of assignment type 1, the switching device may be defective after the short circuit, but station parts or persons must not be endangered (e.g. it must not be possible to touch parts which carry current). In the case of assignment type 2, the switching device must still be capable of functioning after the short circuit, but repairs to the switching device are permitted. For switching devices, fulfillment of assignment type 1 is the minimum requirement.
The stresses which occur in the case of short circuits are brief, abrupt pressure waves, which result from the vaporization of current-carrying parts, above all bonding wires. In the case of a rigid, impermeable housing, these may cause the housing to burst. Approaches to a solution are to make the housing in extremely rigid, strong form—to stand up to the pressure—or to provide the housing with sufficiently large openings—to make it possible to reduce the pressure. The first solution results in very expensive, high-volume constructions. The second solution results in cost-optimized constructions, but often conflicts with other requirements of the product.
In the case of many products, both ways are possible only to a limited extent, so that finally the size of the maximum permitted short circuit current is limited. In the case of the semiconductor power modules considered here, the maximum short circuit currents for bonded modules are significantly less than 50 kA, if the outer wall of the device is very near the wall of the semiconductor module. Greater short circuit currents can only be reached by a sufficiently large distance from the outer wall of the device and sufficient ventilation. Another approach to a solution is to make the outer wall of a very strong material, e.g. steel sheet, which increases the cost and dimensions of the device.